The acquisition of visual-spectrum image data for machine vision inspection of printed circuit boards conventionally has been performed by "moving camera" systems which employ typically a single video camera mounted on a movable staging structure. Such systems are programmed to step the video camera over the circuit board progressively in a serpentine fashion along an x-y coordinate system. The moving camera is stopped and snapped synchronously with strobe light flashes to acquire overlapping images of the entire circuit board at predetermined points along the serpentine coordinate path.
Unfortunately, single moving camera systems prove disadvantageous in the high-volume production of small electronic circuit boards such as those used for automotive engine, message display, transmission and brake controllers due to the unacceptably long cycle times that result from having to continuously move and stop the video camera over the printed circuit board.
Designers of machine vision image data acquisition systems have attempted to reduce the long cycle times of moving camera systems by mounting multiple cameras on a fixed staging structure. Early multiple fixed-camera systems, such as those manufactured by IRI (International Robamation Intelligence) of Carlsbad, Calif., mounted video cameras in four horizontally opposing banks on a large fixed staging structure that required a heavy frame for support. The heavy frame occupied a large footprint on the manufacturing facility floor which contributed to the expense of these systems. The video camera lenses employed had long object distances (approximately 22") and were directed at mirror assemblies oriented 45 degrees to both the cameras and the printed circuit boards. The printed circuit boards were advanced synchronously on the conveyor under the mirror assemblies with snaps of the video cameras. This mirror design disadvantageously inverted the images of the area of interest acquired on the printed circuit board requiring image processing programs to mathematically flip the image back to its actual orientation in space. In addition, the opposing camera and lens configuration made alignment and calibration of the video cameras complex.
Although an increase in cycle time and throughput was achieved in early multiple fixed camera designs, the system resolution was limited to camera arrays having at least 2 mil pixels (0.002 inches per pixel) due to the mounting constraints of the video cameras which had relatively large body sizes. In addition, lenses with long object distances were required to minimize distortion and perspective errors. Complex software, if available, was required to improve inspection performance.
Thus, image data acquisition systems employing multiple fixed cameras with large body or housing dimensions mounted horizontally relative to the conveyor have proven impractical in applications involving relatively small printed circuit boards that are densely populated with small SMD components. Such applications require high system resolution (1 mil per pixel), fast cycle times, and a small system footprint to fit into existing automated manufacturing lines.
Therefore, to reduce cycle time and increase throughput, and to improve data image acquisition quality, a multiple camera image data acquisition system is desired with higher system resolution and cycle times but small physical packaging dimensions.